A traditional cooling structure of an internal combustion engine is known, in which a passage bored along a cylinder of a cylinder block having a plurality of cylinders allows an opening formed in a windward side surface of the cylinder block to communicate with an opening formed in an upper surface of the cylinder head, so that cooling air is guided between the cylinders and to the upper surface of the cylinder head (see e.g. Japanese Utility Model Laid-Open No. Sho 59-005711.
Incidentally, the cooling structure of the internal combustion engine described in Japanese Utility Model Laid-Open No. Sho 59-005711 mentioned above aims to cool a position where the cylinders are adjacent to each other and is a technique of leading cooling air to the upper surface of the cylinder head for efficient introduction of the cooling air. Because of this, cooling of the cylinder head per se has been unconsidered.
The present invention has been made to eliminate such a disadvantage and aims to provide a cooling structure of an internal combustion engine that can improve cooling performance of the engine.